1. Field of Invention
This invention relates generally to protectors for storing overhead transparencies, and more particularly to a protector of this type having a composite structure, the front panel being formed of transparent, moisture-permeable, acetate film whereby when the transparency inserted in the protector is in a moist state, it will dry out in the course of storage.
2. Status of Prior Art
When lecturing before a class, making a business presentation in a board room or some other setting imparting information to an audience, it is common practice to make use of visual aids in the form of transparencies. These transparencies carry images of charts, graphs, drawings, pictures and other forms of data. The image appearing on a transparency is magnified and cast on a large screen viewable by the audience by means of an overhead optical projector.
A typical transparency for use with an overhead projector is a rectangular film 81/2.times.11 inches in size, the film being about 3 mils thick. In order to maintain the transparency in a clean, dust-free condition, it is conventional to jacket the transparency within a transparent envelope or protector made of polyethylene, polypropylene or other synthetic, plastic film material of high clarity and strength.
The transparency is sandwiched between the clear front and rear panels of the protector. The length of the protector is about the same as that of the transparency, but its width is about a half inch greater to define a side margin in which a set of holes is punched so that the protector can be stored in a ring binder or file.
Thus, when a lecture is to be given in a lecture hall, the lecturer brings to the hall a ring binder containing the transparency protectors that are to be used as visual aids in the course of the lecture. Then after being removed from the ring binder, each transparency protector, at the appropriate time, is placed in the rectangular, horizontal stage of the overhead projector so that the projected image of the transparency can be cast onto the screen and viewed by the audience.
The envelope or protector disclosed in the Gardlund Pat. No. 4,402,585 is constituted by a rectangular sleeve of transparent polypropylene film which is open along one longitudinal side edge to permit the insertion of a transparency between the front and rear panel of the sleeve. Holes punched in the longitudinal side edge make it possible to store the protector in a loose leaf binder or a file.
Transparencies carrying images are developed and printed in a photographic processing machine and usually emerge from the machine in a moist state, the transparency then being inserted in a protector to maintain it in a clean, dust-free condition. When the protector is made of Mylar polyester or other high strength synthetic plastic film material which is impermeable to moisture, the moist transparency is unable to dry out unless kept in storage for a prolonged period. As a consequence, if the transparency is still moist when the transparency-loaded protector is put to use in an overhead projector, the residual moisture will impair the clarity of the projected image.
To overcome this drawback, it is known to provide a protector formed entirely of breathable acetate film formed of cellulose acetate resin. One such protector is disclosed in the Hickman U.S. Pat. No. 4,810,544. This protector is formed of a blank of diacetate film that is folded to form the front and rear panels of the protector. The rear panel is of greater width to form a flap that is folded over to form a margin at a side edge of the folder which is provided with a set of punch holes so that the protector can be stored in a ring binder or file.
However, acetate film is relatively weak. Hence, in order to avoid tearing of the protector at the punch holes, adhered to the flap is a strip of double-face, pressure-sensitive tape to reinforce the margins of the protector. A protector of this type is more expensive to make than one made of synthetic plastic film material such as polypropylene, for acetate or diacetate which is formed by two groups of acetate is costly. Moreover, the need to add an adhesive tape strip to the diacetate protector adds substantially to the cost of manufacturing this protector.